Of the different magnetic devices known hitherto and more particularly intended to constitute MRAM memories or radio-frequency oscillators, those said to operate with spin transfer are known. Their operation is based on giant magneto-resistance effects, as for spin valves, and on tunnel magneto-resistance effects, as for magnetic tunnel junctions MTJ.
These structures consist of a stack of magnetic layers, whereof the nature and the arrangement are implemented in such a way that when an electric current passes through them, it is possible to obtain a variable resistance as a function of the magnetic field applied and/or of the spin-polarized current passing through them.
Such a device is constituted by stacking two ferromagnetic layers separated by a non-magnetic layer conventionally known as a spacer, made out of metal for spin valves or oxide for magnetic tunnel junctions.
In a known way, when the direction of magnetization of the two ferromagnetic layers is identical, in what are then referred to as parallel directions, the device is in the low-resistance state. Consequently, when the direction of the two ferromagnetic layers is anti-parallel, the device is in the so-called high-resistance state.
Spintronics uses electron spin as an additional degree of freedom, to generate new effects. Electron spin causes magneto-resistive phenomena in the magnetic multi-layers, such as in particular giant magneto-resistance or tunnel magneto-resistance.
It has in fact been possible to show that by passing a spin-polarized current through a thin magnetic layer, a reversal of its magnetization could be induced in the absence of any external magnetic field. The spin-polarized current may also generate sustained magnetic excitations, also known as oscillations. The use of the effect of generating sustained magnetic excitations in a magneto-resistive device allows this effect to be converted into a high-frequency voltage modulation that can be directly used in electronic circuits, and is therefore as a consequence able to intervene directly at frequency level.
The magneto-resistive stacks providing both said oscillators and memory points for magnetic memories use two different techniques:                so-called “pillar” stacks: all the layers are etched to make a pillar about 50 to 300 nm in diameter;        so-called “contact point” stacks: in a stack of this kind, the active and particularly ferromagnetic layers are not etched with nanometric patterns or, if they are, are then manufactured in accordance with very large patterns (typically in the vicinity of a square micrometer). A very tight metal contact is made, typically 20 to 50 nm, above the magnetic stack using an external or internal nanotip.        
Stacks of the second aforementioned type, also called “nano-contacts” are preferred, particularly when spin valves are employed in order to make radio-frequency oscillators, since they produce better defined radio-frequency emissions and particularly sharper radio-frequency emissions. It has in fact been possible to observe a reduction in the width of the radio-frequency emission lines, a reduction attributed to the minimization of the edge effects inherent in the method of manufacture.
Moreover, using nano-contacts means that the integration density of the magnetic devices can be increased and the current can be confined in the magnetic layers allowing a homogenization of the physical effects, as well as a reduction in bipolar interference fields. In the particular context of producing radio-frequency oscillators, the use of nano-contacts allows very high Q=f/Δf quality factors to be obtained.
In fact, radio-frequency oscillators of this kind are, as has already been said, more particularly employed in the context of telecommunications devices, such as in particular cell telephones in respect of which more and more work is being done on applying the dynamic frequency allocation principle to resolve the problem caused by the saturation of the frequency bands assigned to the telecommunications sector. And, to implement said dynamic frequency allocation principle, very wide band oscillators are needed that offer very good phase noise performance, and consequently require a very high quality factor Q.
Technologically speaking, nano-contacts of this kind cannot be put into mass production using current technology. Said technology in fact demands tools that are very cumbersome to use, such as an FIB (focused ion beam) and electroplating, AFM (Atomic Force Microscope) tips to make nano-indentations, in other words a method for making orifices one by one in a planarizing resin, EBEAM (electron beam)/metal etching/planarization or again EBEAM/oxide etching/filling photolithography sequences, particularly when it is required to obtain contact dimensions well below 50 nm.
The objective targeted by the present invention is to make nano-contacts of this kind in magnetic stacks, and to do so, using another technology, that can be more easily adapted to mass production and allows component integration to be optimized.